[ULPIANUS libro secundo disputationum. ] §15.3.15.prSi filius familias constituerit quod pater debuit, uidendum est, an de in rem uerso actio dari debeat.
[ULPIAN, Disputations, book 2] If a son-in-power has promised to pay what his father owed, it must be considered whether an action for what was turned to account should be given.
atquin non liberauit patrem: nam qui constituit, se quidem obligat, patrem uero non liberat.
And yet, he has not released his father; for he who promises to pay indeed binds himself, but does not release his father.
plane si soluat post constitutum, licet pro se uideatur soluisse, hoc est ob id quod constituit, in rem tamen uertisse patris merito dicetur.
Clearly, if he pays after the promise, although he may seem to have paid on his own behalf, that is, on account of what he promised, it will nevertheless be rightly said to have been turned to the account of the father.